This invention relates to the recovery of gold from ores and, more particularly to an improved pressure oxidation process for the recovery of gold from refractory ores.
In order to remove sulfide sulfur, refractory ores are conventionally treated by pressure oxidation before cyanide leaching. If the sulfide sulfur is not substantially oxidized, leaching is inhibited and gold remains locked in the sulfides. By treating the ore in an aqueous slurry at elevated temperature and oxygen pressure, the sulfur is oxidized and removed from the ore before it is contacted with cyanide leaching agent. Thereafter the gold is leached by the cyanide and acceptable yields are produced.
Pressure oxidation is an exothermic process but requires the use of a substantial amount of energy in pre-heating the ore slurry to a temperature at which the reaction is self-sustaining. The oxidized slurry may contain substantial amounts of iron, arsenic and other heavy metals which it is desirable to remove before cyanidation. These various metals are typically oxidized during the pressure oxidation step, but further measures are required if the salts and oxides of these undesired metals are to be removed from the process.
Weir et al U.S. Pat. No. 4,571,263 describes a process for pressure oxidation of refractory ores in which the effluent from the pressure oxidation autoclave is subjected to a two step repulping operation with solids-liquid separations after each step. Liquid from the second separation step is recycled to the first repulping step. Liquid from the first separation step is in part recycled to pressure oxidation and in part subjected to a two step precipitation first with limestone and then with lime. Effluent slurry from the second precipitation step is subjected to solids-liquid separation and the liquid fraction is passed through a cooling pond and recycled to the second repulping step and the pressure oxidation step.
Weir U.S. Pat. No. 4,571,264 describes a pressure oxidation gold recovery process in which the effluent from the pressure oxidation step is repulped, thickened and then subjected to a two stage washing process. Water for washing derives from a liquid fraction produced in thickening the ore slurry after acid pretreatment prior to pressure oxidation. This liquid fraction is subjected to a two stage precipitation with limestone and lime, respectively. The effluent slurry from the second precipitation is thickened, and the liquid overflow from the latter thickener is used as water in the second washing stage. A solids-liquid separation after the second washing stage produces a liquid fraction which is recycled and serves as the wash water in the first washing stage.
In both Weir et al '263 and Weir et al '264, the neutralized oxidized slurry is subjected to cyanidation, followed by an eight stage carbon-in-leach absorption process. Both patents disclose pressure oxidation at 160.degree. to 200.degree. C. and 700-5000 kPa (total pressure).
Weir U.S. Pat. No. 4,606,763 describes pressure oxidation at 165.degree. C. and 50-2000 kPa total pressure, using a compartment autoclave in which the first compartment is approximately twice the size of each of the other compartments. Weir U.S. Pat. No. 4,605,439 discloses a pressure oxidation process operated at 120.degree. to 250.degree. C. and 350-6000 kPa. Mason et al U.S. Pat. No. 4,552,589 discloses alkaline pressure oxidation at 220.degree.-250.degree. C. and 10-25 psia oxygen partial pressure for 30 to 90 minutes. Matson et al U.S. Pat. No. 4,289,532 describes alkaline pressure oxidation at 140.degree.-190.degree. F. using air.